Maps Marker is one of the best non-commercial plugins I have had the pleasure to use. It has a wealth of features, of which I have barely scratched the surface, and if I had one criticism it would be that the sheer number of options available is overwhelming.
Maps Marker has allowed me not just to integrate location based information on my site, but also has great integration with the Wikitude augmented reality software which I have taken full advantage of. It is rare to find a plugin that not only meets your expectations, but surpasses them!
If you want a simple mapping tool this may not be for you, but if you need plenty of options in how your information is presented and the ability to 'tinker under the hood' then this is by far the best choice for a mapping plugin.